One form of cytochrome P450 from 3-methylcholanthrene-induced rat liver, P450-reductase and epoxide hydratase have each been purified to homogeneity, and metabolism of benzo(a)pyrene (BP) has been reconstituted with these component enzymes. UDP-glucuronic acid via action of UDPGA-glucuronyl transferase effects enhancement (2.5x) of the formation of BP-7,8 dihydrodiol 9,10 oxides from BP and the ensuing modification of DNA. This activation is due to removal via conjugation mechanisms of BP-quinones which are potent non-competitive inhibitors of BP-metabolism and, more potently, BP-7,8 dihydrodiol metabolism. Metabolism of BP in hepatocytes from MC-induced rats resulted in modification of DNA which was highly dependent both on the amount of added BP and the medium. Modification of guanine by BP-dihydrodiol epoxides was only detectable at BP concentrations of 15 micron M with a cell concentration of 10 to the 6th power/ml. At 60 micron M BP, modification increased 10x. Modification was 4x greater in L15 medium than in Krebs-Ringer bicarbonate buffer. Modification of DNA by BP-phenol metabolites was only observed when free-SH groups were depleted by diethyl maleate. Products from BP-metabolism in 10 T1/2 mouse fibroblasts (transformed by BP) were indistinguishable from those in CVP mouse fibroblasts (non-transformed). Metabolic rate and cytochrome P450 were higher in 10 T1/2 cells and increased (hydrocarbon-induced) during log phase growth reading a maximum at confluence.